Researchers have exploited defects in two dimensional materials to dramatically enhance the conversion of temperature differences into electricity.

© Wu Jing

The defects that spark joy

16 Dec 2020

Natural imperfections trigger a Kondo effect in 2D materials that bring their thermoelectric performance to new heights.

You’re probably thinking of the wrong Kondo. Unlike how TV personality and author Marie Kondo keeps closets spotless and tidy, new research suggests that in thermoelectric materials, embracing and taking advantage of impurities—the Kondo effect—could show the way forward.

The thermoelectric caliber of any given material—that is, how well it can convert a temperature difference into electric power—often depends on three factors: the Seebeck coefficient, which is a measure of the temperature-induced voltage build-up, and its electrical and thermal conductivities. Ideal thermoelectric materials need to exhibit a high Seebeck coefficient and electrical conductivity, while simultaneously having low thermal conductivity.

The problem is that the Seebeck coefficient and electrical conductivity sit on opposite ends of a see-saw—when one rises, the other drops. Such an interdependency puts a cap on thermoelectric performance, as too much heat is needed to generate an underwhelming amount of power.

To break through this anti-correlation ceiling, a team including first author Wu Jing and co-corresponding author Kedar Hippalgaonkar, both Research Scientists at A*STAR’s Institute of Materials Research and Engineering (IMRE), looked at naturally occurring defects in a two-dimensional stack of n-type molybdenum disulfide (MoS2) supported on a hexagonal boron nitride (h-BN) substrate.

These defects, which are caused by missing sulfur atoms within the MoS2/h-BN lattice, could be observed using a technique called low temperature-scanning tunneling microscopy. “Thanks to our colleagues Yanpeng Liu and Kian Ping Loh at the National University of Singapore, we were able to obtain high-resolution images of the defects, which helped to isolate the cause of our surprising results,” Hippalgaonkar said.

The researchers propose that these sulfur vacancies could act like magnetic impurities and, at relatively low temperatures, trigger a Kondo effect in the MoS2 flakes—scattering its conduction electrons, altering its band structure, and amplifying its thermoelectric properties.

Indeed, when warmed to only around 60 K, Kondo scattering started to dominate the thermoelectric transport in the flakes. In turn, this drove the Seebeck coefficient up to an extremely large peak value of ~2 mV/K. At the same time, interactions between the electrons and impurities gave rise to a Kondo resonance effect, leading to an anomalous reversal in the Seebeck coefficient’s sign, from negative to positive.

“Our MoS2/h-BN sample can exhibit both negative Seeback values due to its n-type nature, and positive Seebeck values by inducing band hybridization,” Jing said. “These results suggest that a singly doped material could be used to fabricate thermoelectric devices such as nanoscale cooling devices.”

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of High Performance Computing (IHPC).

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Wu, J., Liu, Y., Liu, Y., Cai, Y., Zhao, Y., et al. Large enhancement of thermoelectric performance in MoS2/h-BN heterostructure due to vacancy-induced band hybridization, PNAS 117, 13929-13936 (2020). | article

About the Researchers

Kedar Hippalgaonkar

Research Scientist

Institute of Materials Research and Engineering
Kedar Hippalgaonkar is a Research Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), and an Assistant Professor at the Department of Materials Science and Engineering, Nanyang Technological University, Singapore. Hippalgaonkar received his Bachelor’s degree in Mechanical Engineering from Purdue University, and his PhD degree in Mechanical Engineering from the University of California, Berkeley. At IMRE, he leads a research group that focuses on heat, charge, and thermoelectric transport, as well as the interactions between phonons, photons, and electrons in 1D, 2D, and inorganic-organic hybrid materials.

Jing Wu is a Research Scientist at the A*STAR’s Institute of Materials Research and Engineering (IMRE). He received his Bachelor’s degree in Physics from Zhejiang University (China) in 2010 and Ph.D. degree from the physics department at National University of Singapore (NUS) in 2015. His current research interests mainly focus on heat and thermo/opto-electronic transport in novel 2D materials.

This article was made for A*STAR Research by Wildtype Media Group